Secure communications systems are known in the art. Such systems typically render a voice message unintelligible to prevent unauthorized reception. To accomplish this the voice message can be digitized and processed through an encryption device to produce a resulting signal that appears to be random (or pseudo-random) in nature. Such a signal appears like noise to unauthorized receivers and discourages intelligible reception. The particular encryption algorithm used by the encryption device maybe a proprietary algorithm or may be based on a standard such as the Data Encryption Standard (DES) promulgated by the United States National Bureau of Standards. In a typical encryption device a secret key is utilized in conjunction with a cipher algorithm to encrypt and decrypt messages. The keys may be changed or rekeyed as frequently as desired in order to enhance security.
It is sometimes however desirable in such voice protection or secure systems to maintain different levels of security. A supervisor for example may wish to deliver a scrambled message to one user or a set of users without communicating with another set of users on the same channel. One method of accomplishing this is to call a remote unit in the clear (without encryption) and identify a particular key that is to be used to unscramble an ensuing message. Key identification is used to identify which one of a multiplicity of keys was used to encrypt the message and also to keep the access time down to a minimum. This however requires vast memory storage of the keys in the subscriber unit especially if the keys are to be unique and are to be changed frequently.
Such secure communication is a feature mainly available on conventional communications systems that make use of dedicated channels. This has occurred in part because the encrypted signal itself comprises a 12 kilo bits per second (KBS) data string that requires substantially all of the available spectrum of the assigned channel. Currently conventional communications systems use a secure communication method which is performed by sending encrypted information about the key to be used via signalling data. This key information includes the transmission of the actual key having multiple (64) bits for DES.
Conventional channel allocation systems however do not represent optimum usage of the increasingly crowded communications spectrum. Trunked systems are well recognized to make more efficient usage of available channel allocations. Such systems typically include at least one central controller or fixed end that controls channel allocation between various subscriber units (as used herein subscriber units includes all remote transceiving devices such as mobile units installed in vehicles, other controlled stations, portable devices, and radio frequency (RF) linked telephones).
However, far too much information is sent using the conventional remote key information method for it to be possible to use the control channel of a trunked system. Especially when a fleet of subscriber units or radios have to be rekeyed, a lot of air time would be consumed on the control channel in a trunked system, rendering it effectively immobilized for quite some time. Such a signal thus presents compatibility problems when compared to the trunked channel maintenance protocol which includes control signals such as connect tones, handshake signals, acknowledge tones, and disconnect tones.
Accordingly there exists a strongly felt need for a combined secure trunked communication system that is remotely rekeyable without taking up too much system access time or subscriber memory space.